Charmonium (and Open-Charm) production
at the CERN- SPS
P. Cortese
Università del Piemonte Orientale and INFN – Alessandria, Italy
NA60 Collaboration
International Workshop on Heavy Quark Production
in Heavy-ion Collisions / January 4-6, 2011
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Introduction
Charmonium suppression in p-A and A-A collisions
NA60 results on thermal dileptons
Preliminary results on the A-dependence of the open charm yield
1
THE hard probe at SPS energy
• Study of charmonium production/suppression in pA/AA collisions
AA collisions
Color screening and charmonium suppression
> 20 year long history
pA collisions
Initial/final state nuclear effects (shadowing, dissociation,...)
Reference for understanding dissociation in a hot medium
Production models (CSM, NRQCD, CEM, ....)
2
Experimental landscape
(Relatively) large amount of fixed-target data (SPS, FNAL, HERA)
AA collisions
NA38
NA50
NA60
(M.C. Abreu et al., PLB449(1999)128)
S-U 200 GeV/nucleon, 0<y<1
(B. Alessandro et al., EPJC39 (2005)335)
Pb-Pb 158 GeV/nucleon, 0<y<1
(R. Arnaldi et al., PRL99(2007) 132302)
In-In 158 GeV/nucleon, 0<y<1
pA collisions
NA50
NA3
NA60
E866
HERAB
(B. Alessandro et al., EPJC48(2006) 329)
p-Be,Al,Cu,Ag,W,Pb,400/450 GeV,-0.1<xF<0.1
(J. Badier et al., ZPC20 (1983) 101)
p-p p-Pt, 200 GeV, 0<xF<0.6
(E. Scomparin et al., nucl-ex/0907.3682, R. Arnaldi et al., nucl-ex/0907.5004)
p-Be,Al,Cu,In,W,Pb,U 158/400 GeV,-0.1<xF<0.35
(M. Leitch et al., PRL84(2000) 3256)
p-Be,Fe,W 800 GeV,-0.10<xF<0.93
(I. Abt et al., arXiv:0812.0734)
p-Cu (Ti) 920 GeV,-0.34<xF<0.14
3
The NA38 and NA50 experiments
Centrality detectors
• EM calorimeter (NA38-NA50)  ET
• ZDC (NA50)
• Multiplicity detector (NA50)
working also as an interaction detector
Coverage:
• 2.92<ylab<3.92
• -0.5<cosqCS<0.5
Typical acceptances:
• for J/y ~ 14%
• for y’ ~ 15%
Muon spectrometer
• Hadron absorber
• Air-core toroidal magnet
• MWPCs
• redundant trigger system
4
NA50 upgrades along the years
Experimental setup
1995-1996
Thick segmented target
1998
Thin target to avoid ion
reinteractions
2000
Thin target under vacuum
to eliminate Pb-Air
parasitic interactions
Target identification in
data analysis
1995-1996
with interactions detectors
1998-2000
Hit correlation between multiplicity
detector planes
5
The reference for J/y data: Drell-Yan
Advantages
 Unambiguous extraction from the invariant
mass fit
Drell-Yan
 Same systematics as the J/y (trigger, rec.
efficiency, selection criteria)
 Cancellation of eventual biases in the ratio
 (Drell-Yan)=1
Different production mechanism (q-q vs. g-g) but
at SPS energies and in the NA50 kinematic
domain shadowing is small
 (ψ)/(DY) proportional to (ψ)/NColl
Drawback
 Drell-Yan cross section lower than J/ψ
 Error on DY dominates in the ratio
6
J/y suppression in Pb-Pb vs centrality
Nuclear modification factor
ψ/DY in p-p
Good agreement
between the two golden
data sets
Strong suppression of ψ
yield going from
peripheral to central
collisions
Pb-Pb at 158 A*GeV
Peripheral collisions
… without estimating the
cold-nuclear-matter
reference little can be
said about hot nuclear
matter effects
Central collisions
7
Measuring CNM reference in p-A
450 GeV
abs=6.91.5
NA38 Collaboration
PLB 444 (1998) 516
First determination (problematic)
SPS A-A data has been taken at ~Z/A*400 GeV incident momentum
S and O induced reactions at 200 A*GeV
Pb-Pb at 158 A*GeV (and also In-In for consistency)
p-A at 400 (or 450 GeV)
Only recently CNM reference has been evaluated with 158 GeV
primary proton beam by NA60
8
Re-measuring CNM effects: 1996-2000
abs=4.30.7 for HI 97/98
abs=4.41.0 for LI 98/00
All+NA51
abs=3.41.2 for HI2000
3 data sets in different conditions
Use ψ/DY to reduce possible systematics
Updates in analysis procedures
Results are consistent and indicate lower absorption
9
Comparing with lower energy data
• Estimate the normalization of CNM reference at 158 GeV
• To do this had to assume that CNM effects do not depend on
energy
p-A data at lower energy are compatible (large errors!)
Could also assume that only CNM effects are present in S-U 200 GeV
10
Extrapolating to A-A
Assuming energy independence of CNM effects…
…in p-A collisions
…in p-A collisions
+ no hot-nuclear
matter effects in
S-U
Larger error but minimal assumptions
11
Anomalous J/ψ suppression
Bµµσ(J/ψ)/σ(DY 2.9-4.5) Pb-Pb results do not follow the
normal absorption as extrapolated from p-A
Peripheral collisions:
Compatible with the
normal nuclear absorption.
Mid–centrality:
Departure from the normal
nuclear absorption.
Central collisions:
No saturation at high
centralities.
12
Other centrality estimators
Same behavior is observed using charged particle multiplicity
or zero degree energy as centrality estimators
13
Ψ’ production
p-A: path in nuclei  possibility to constrain the
production mechanisms
A-A: different binding energy  sensitivity to
temperature of plasma or hadronic soup
Higher absorption for ψ’
Confirms E866 observation
14
Ψ’ production in p-A and A-A
Suppression in p-A
extrapolated to A-B assuming
energy independence of abs
Different behaviour
between p-A and A-B
collisions
Independently from
assumptions on abs
Stronger ψ’ absorption from
peripheral to central A-B
interactions.
ψ’ suppression in S-U
collisions is compatible with
Pb-Pb
15
The NA60 experiment
NA60, the third generation experiment studying the production of
muon pairs at the CERN SPS
2.5 T dipole magnet
Muon trigger and tracking
NA10/38/50 spectrometer
targets
Matching in coordinate
and momentum space
hadron absorber
Iron wall
magnetic field
vertex
tracker
beam
tracker
Muon
Other
Data samples
• In-In collisions at 158 GeV/nucleon
• p-A collisions at 158 and 400 GeV
• 9 nuclear targets, Al-U-W-Cu-In-Be1-Be2-Be3-Pb
(mixed A-order to limit possible z-dependent systematics) 16
p-A collisions
Not enough DY statistics to extract
(as in NA50) B J/y/DY target by
target
W
Pb
Cu
In
U
Estimate of nuclear effects through
relative cross sections:
 AJ /y
J /y
 Be
N AJ /y
inc
N A  N At arg  AA   A

J /y
N Be
inc
t arg
N Be  N Be
 ABe   Be
Al
Be
all targets simultaneously on the beam
 beam luminosity factors Niinc cancel out
(apart from a small beam attenuation
factor)  no systematic errors
 acceptance and recostruction efficiencies do not completely cancel out
(these quantities and their time evolution are computed for each target)
 each target sees the vertex spectrometer under a (slightly) different
angle: kinematic window is restricted, to reduce systematic
(0.28<y <0.78 at 158GeV and -0.17<y <0.33 at 400GeV)
17
A-dependence of relative cross sections
400 GeV
 increasing suppression of
the J/y yield as a function of A
 larger suppression at 158 GeV
using a Glauber fit to
extract abs
158 GeV
NA60 Coll., arXiv:1004.5523
systematic errors due to :
• target thicknesses (<1.5%)
• J/y y distribution (<1.5%)
• reconstruction eff. calculation (<3%)
shadowing neglected, as usually done
at fixed target (but not correct!)
158 GeV:
abs J/y = 7.6 ± 0.7 ± 0.6 mb
400 GeV:
abs J/y = 4.3 ± 0.8 ± 0.6 mb
very good agreement with
NA50 (4.6 ± 0.6mb)
using
 pA   pp A
158 GeV:
 = 0.882 ± 0.009 ± 0.008
400 GeV:
 = 0.927 ± 0.013 ± 0.009
18
Shadowing at 400/158 GeV
• We have evaluated (and corrected for) the (anti)shadowing effect
expected for our data points, within the EKS98 and EPS08 scheme
158 GeV,
EKS98
EPS08
The Glauber fit now gives
abs
abs
abs
abs
400 GeV,
EKS98
EPS08
(158 GeV)=9.3±0.7±0.7 mb
J/y,EPS (158 GeV)=9.8±0.8±0.7 mb
J/y,EKS (400 GeV)=6.0±0.9±0.7 mb
J/y,EPS (400 GeV)=6.6±1.0±0.7 mb
J/y,EKS
Significantly higher than the “effective” values
19
 vs xF
• NA60 pA results can be compared with  values from other
experiments (HERAB, E866, NA50 and NA3)
E. Scomparin et al., Nucl. Phys. A830 (2009) 239
In the region close to xF=0,
increase of  with √s
NA60 400 GeV
 very good agreement
with NA50
NA60 158 GeV:
 smaller , hints of a
decrease towards high xF ?
 discrepancy with NA3 ?
Systematic error on  for NA60 points ~0.01
20
 vs x2
• Shadowing effects and final state absorption (in the 21 approach)
should scale with x2
• If parton shadowing and final
state absorption were the only
relevant mechanisms
  should not depend on √s at
constant x2


x2  mT / s e  y

• NA60 data (two energies in the
same experimental conditions)
 reduced systematics on 
NA60 Δα(400-158 GeV)
  0 in the explored x2 range
 clearly other effects
should be present
x2
NA60 Coll., arXiv:1004.5523
21
Reference for AA data
• CNM effects, evaluated in pA, can be extrapolated to AA, assuming a
scaling with the L variable and taking into account that:
abs shows an energy/kinematical
dependence
reference obtained from 158 GeV pA data (same
energy/kinematical range as the AA data)
in AA collisions, shadowing
affects both projectile and target
proj. and target antishadowing taken into
account in the reference determination
The current reference is based on:
• slope determined only from pA@158GeV
abs
J/y
(158 GeV) = 7.6 ± 0.7 ± 0.6 mb
• normalization to J/ypp determined from
pA@158 GeV (J/y/DY point) and (to
reduce the overall error) SU@200GeV
SU has been included in the fit, since it has a
slope similar to pA at 158 GeV
advantage: small error on normalization (3%)
drawback: hypothesis that SU is “normal”
22
New result: J/y cross section in pA
• J/y production cross sections for pA data
Preliminary
• Systematic error on (absolute) luminosity estimation quite high
• Relative luminosity estimate between 158 and 400 GeV
much better known (~2-3% systematic error)
• Normalize NA60 400 GeV cross section ratios to NA50 results
• 158 GeV cross sections constrained by the relative normalization
23
New reference using J/y cross sections
• Alternative approach for the normalization of the pA reference
curve based on the pA J/y absolute cross section
• To fully profit from this approach, a measurement of the absolute J/y
cross section in In-In would be needed. For the moment…
• J/y/DY values are obtained rescaling the DY cross section
measured at 450 GeV by NA50 (not enough statistics at 158 GeV)
• Main advantage: no assumption on SU, since it is not used
anymore in the fit
Difference with previous CNM
reference ~1% well within
errors
Preliminary
No practical consequence on
anomalous J/Ψ suppression
Now possible to claim that in
SU are present only CNM
effects
24
Anomalous suppression
In-In 158 GeV (NA60)
Pb-Pb 158 GeV (NA50)
Using the previously
defined reference:
Central Pb-Pb:
 still anomalously
suppressed
In-In:
 almost no anomalous
suppression
B. Alessandro et al., EPJC39 (2005) 335
R. Arnaldi et al., Nucl. Phys. A830 (2009) 345
R.Arnaldi, P. Cortese, E. Scomparin Phys. Rev. C 81 (2009), 014903
25
Anomalous suppression: SPS vs RHIC
• SPS results compared with RHIC RAA results normalized to RAA(CNM)
 Both Pb-Pb and Au-Au seem to
depart from the reference curve
at Npart~200
 For central collisions more
important suppression in
Au-Au with respect to Pb-Pb
M. Leitch (E866), workshop on “Quarkonium in
Hot Media:From QCD to Experiment”, Seattle 2009
• Agreement between SPS and RHIC
results as a function of the charged
particle multiplicity
26
Charmonia summary
• J/ψ effective absorption cross section in p-A shows energy
dependence
• different  (158 vs. 400 GeV) at the same x2
• effect not related to shadowing or s(ψ-nucleon)
• New reference for Pb-Pb and In-In based on p-A at the same
energy
• Size of anomalous J/ψ suppression reduced but still
significant in Pb-Pb
• ψ’ more absorbed than J/ψ
• Strong ψ’ suppression in Pb-Pb
• Similar L pattern in Pb-Pb and SU
27
Open charm dimuons in p-A 450 GeV
• NA50 tried to evaluate DD production studying the IMR in pA
• DD pair detected with simultaneous semi-muonic decay
High beam intensities
Large background levels (S/B ~0.05 at m = 1.5 GeV/c2)
NA50 had to impose
a constant DD/DY vs A
(i.e. DD= DY ~1 )
M.C. Abreu et al., EPJC14(2000) 443
28
Charm cross section at √s 29.1 GeV
Open charm
Good description of
the invariant mass
spectra as sum of DD
and DY
Drell Yan
Open charm cross
section in agreement
with world
systematics
29
Dimuon excess wrt p-A
Pb-Pb peripheral
Pb-Pb central
• Excess mass shape and kinematical
distributions compatibile with open
charm
• Possible open-charm enhancement
• Alternative explanation: direct
radiation (from the plasma?)
30
NA60 In-In: intermediate mass region
Eur.Phys.J. C59 (2009)
607
Mass spectrum is similar to NA50:
Good description by Drell-Yan + ~2Open
Charm (extrapolated from pA data)
DY
Such explanation is rejected by the spectra of
dimuon offsets wrt the interaction vertex!
   ( x 2 Vxx1   y 2 Vyy1  2 x  y Vxy1 ) / 2
   (21  2 2 ) / 2
Fit range
DD
DD
Prompt
~50m
DD
Data
Prompt:
1.120.17
2.290.08
Charm:
1.160.16
Fit 2/NDF: 0.6
~1mm
Offset fit shows that the enhancement is not
due to Open Charm  the excess is prompt
Prompt
31
NA60 In-In: low mass region
Clear excess of data above decay
‘cocktail’ describing peripheral events
Phys. Rev. Lett. 96 (2006) 162302
•Excess isolated subtracting the measured decay cocktail
(without r), independently for each centrality bin
•Based solely on local criteria for the major sources:
η, ω and f  2-3% accuracy.
 No need of reference data (pA, peripheral data, models)
 Less uncertainties (e.g. strangeness enhancement: ,f)
32
Effective temperature of excess dileptons
M<1 GeV
thermal dilepton production largely
mediated by the broad vector meson
ρ via pp→r→g→ annihilation.
Hadronic nature supported by the
rise of radial flow up to M=1 GeV
M>1 GeV
sudden fall of radial flow of thermal
dimuons occurs, naturally explained
as a transition to a qualitatively
different source, i.e.
mostly partonic radiation, qq→g→μμ
Phys. Rev. Lett. 96 (2006) 162302
Phys. Rev. Lett. 100 (2008) 022302
33
Polarization of excess dileptons
dσ



 1   cos2 q   sin 2q cos f  sin 2 q cos 2f 
 d cosq df 
2

1
Analysis in CS reference
frame. Same results in
Gottfried Jackson reference
frame
Lack of any polarization in excess supports
emission from thermalized source.
34
Open charm production in p-A collisions
• Open charm shares initial state effects with charmonium
 a measurement of open charm in p-A collisions may help
in understanding J/y suppression
• Recent results from SELEX and E866 suggest rather strong
nuclear effects on open charm
E866/NuSea Preliminary
A. Blanco et al. (SELEX), EPJC64(2009) 637
M. Leitch (E866), workshop on “Heavy Quarkonia
Production in Heavy-Ion Collisions”, ECT* 2009
35
Open charm dimuons in p-A: NA60
• High-mass DY statistics is low in NA60
Use the ratios y/DY from NA50 (EPJC48(2006)329)
to fix the DY contribution
400 GeV
DD
DY
• Good resolution on the
longitudinal position of the
vertex in the IMR (no problem
in target assignment)
• Results given for the 400 GeV
sample only (at 158 GeV DD
contribution much smaller
than DY)
36
Open charm dimuons in p-A: results
• Kinematic range: 3.2<ylab<3.7 (-0.17<ycm<0.33)
• Study the ratio y/DD (reduce systematic errors)
Preliminary
• Systematic errors
•
•
•
•
Fit starting point
Track 2
y/DY (norm. and J/y)
Background normalization
 y / DD  0.97  0.03 stat   0.01syst 
 y    DD    0.03  0.03 stat   0.01syst 
• Using the measured J/y value one gets DD = 0.960.03
• Anti-shadowing would suggest DD >1
37
x2
x2
DD (PYTHIA)
pA 400 GeV
(3.2<y<3.7)
Calculate the expected
 (pure shadowing) for
DD pairs and J/ψ
decaying into muons
in the NA60
acceptance (400 GeV
protons)
J/y
(21 calculation)
pA 400 GeV
(3.2<y<3.7)
xx11
x1
Anti-shadowing would give:
• approximately the same  for
DD and J/y
• >1
Data suggests:
• approximately the same 
• <1

Preliminary
DD MC
J/ψ MC
DD data
J/ψ data
Conclusions
• NA60 performed detailed studies of charmonium suppression in pA
• Nuclear effects stronger when decreasing √s
• Build CNM reference
• with pA data taken in the same energy/kinematic
domain of AA
• taking into account shadowing
• Look for anomalous suppression in In-In and Pb-Pb
• Observed in central Pb-Pb collisions
• In In-In almost no anomalous suppression
• With updated (correct) CNM reference seeming agreement between
SPS and RHIC J/ψ data when studied as a function of dNCH/dη
• Open charm in pA can be studied looking at the IMR dimuons
• Preliminary results for pA at 400 GeV: DD close to 1
39
The NA60 collaboration
http://cern.ch/na60
CERN
Heidelberg
~ 60 people
13 institutes
8 countries
Bern
Palaiseau
BNL
Riken
Yerevan
Stony Brook
Torino
Lisbon
Clermont
Lyon
Cagliari
R. Arnaldi, R. Averbeck, K. Banicz, K. Borer, J. Buytaert, J. Castor, B. Chaurand, W. Chen, B. Cheynis,
C. Cicalò, A. Colla, P. Cortese, S. Damjanović, A. David, A. de Falco, N. de Marco, A. Devaux, A. Drees,
L. Ducroux, H. En’yo, A. Ferretti, M. Floris, P. Force, A.A. Grigoryan, J.Y. Grossiord, N. Guettet, A. Guichard,
H. Gulkanyan, J. Heuser, M. Keil, L. Kluberg, Z. Li, C. Lourenço, J. Lozano, F. Manso, P. Martins, A. Masoni,
A. Neves, H. Ohnishi, C. Oppedisano, P. Parracho, P. Pillot, T. Poghosyan, G. Puddu, E. Radermacher,
P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan,P. Sonderegger, H.J. Specht,
R. Tieulent, E. Tveiten, G. Usai, H. Vardanyan, R. Veenhof and H. Wöhri
40
40
Invariant mass spectra and fits
• Fit the reconstructed invariant mass spectrum as a superposition
of the various expected sources: Drell-Yan, J/y, y’, open charm
DY
J/y, y’
DD
PC muons,
all targets
J/y statistics,
after analysis
cuts
VT muons,
Pb target
158 GeV, NJ/yPC = 2.5104, NJ/yVT= 1.0 104
400 GeV, NJ/yPC = 1.6104, NJ/yVT= 6.0 103
41
41
Efficiency corrections
Matching rate data
• Reconstruction efficiency does not cancel in the ratio
• Estimation based on a Monte-Carlo approach
• Inject realistic VT efficiencies (follow their time evolution)
• Finest granularity (8 pixels ~0.8*0.8 mm2 where statistics is enough)
• Use “matching efficiency” and its time evolution as a check of
the goodness of the procedure
Matching rate MC
Efficiency map
(4th plane, sensor 0)
Matching efficiency
42
42
Other pA results: pT distributions
NA60 158 GeV
<pT2>= <pT2>pp+ r (A1/3-1)
R. Arnaldi et al., Nucl. Phys. A830 (2009) 345
•
•
•
•
pT broadening observed (usually ascribed to Cronin effect)
NA60 results suggest a smaller broadening at low √s
NA3 result similar to higher √s values
agreement NA60 vs NA50 at 400 GeV
43
Differential distributions: dN/dy
158 GeV
400 GeV
• y-distribution wider at 400 GeV, as expected
• Gaussian fit at 158 GeV gives μy=0.05±0.05, σy=0.51±0.02 (in agreement with NA3)
• Peak position not well constrained at 400 GeV
44
• Imposing μy=-0.2 (NA50 at 400 GeV) σy=0.81±0.03 (NA50 got 0.85)
44
Comparison with previous experiments
• HERA-B observes, at 920 GeV, a displacement of the center of
the xF distribution towards negative values, increasing with A
(by a small amount, xF< 0.01)
HERA-B
NA50
• NA50 observes, at 400 GeV, a strong backward displacement
(y=0.2, corresponding to xF= 0.045)
• Mutually incompatible observations? NA60 data not precise
enough to discriminate between the two scenarioes
45
45
Other pA results: polarization
dσ



 1   cos 2 q   sin 2q cos f  sin 2 q cos 2f 
 d cosq df 
2

1
Helicity
 compatible with zero
everywhere
no large differences between
NA60, HERAB and PHENIX
as observed by HERAB,
|λHE|< |λCS|
HE
CS
R. Arnaldi et al., Nucl. Phys. A830 (2009) 345
p-A 158 GeV
46
Other AA results: polarization
• First full measurement of the J/y angular distribution in nuclear collisions
vs. pT
vs. centrality
Polarization is rather
small everywhere: no pT
or centrality dependence
Positive azimuthal
coefficient at low pT?
Polarization in AA may
be influenced by the
hot medium
 quantitative
predictions needed!
R. Arnaldi et al.,
Nucl. Phys. A830 (2009) 345
47